Durham Research Online - COnnecting REpositories · 2013-09-13 · Zagros Simply Folded Belt in the Fars region, and ~120km total shortening of ... et al., 2012]. The present day

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Allen, M.B. and Saville, C. and Blanc, E.J-P. and Talebian, M. and Nissen, E. (2013) ’Orogenic plateaugrowth : expansion of the Turkish-Iranian Plateau across the Zagros fold-and-thrust belt.’, Tectonics., 32 (2).pp. 171-190.

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Orogenic plateau growth: Expansion of the Turkish-IranianPlateau across the Zagros fold-and-thrust belt

M. B. Allen,1 C. Saville,1 E. J-P. Blanc,2 M. Talebian,3 and E. Nissen4

Received 9 July 2012; revised 27 November 2012; accepted 3 February 2013; published 26 April 2013.

[1] This paper shows how the Turkish-Iranian Plateau grows laterally by incrementallyincorporating adjacent parts of the Zagros fold-and-thrust belt. The limit of significant,seismogenic, thrusting in the Zagros (Mw> 5) occurs close to the regional 1250melevation contour. The seismicity cutoff is not a significant bedrock geology boundary.Elevations increase northward, toward regional plateau elevations of ~2 km, implying thatanother process produced the extra elevation. Between the seismogenic limit of thrustingand the suture, this process is a plausibly ductile thickening of the basement, suggestingdepth-dependent strain during compression. Similar depth-dependant crustal strain mayexplain why the Tibetan plateau has regional elevations ~1500m greater than the elevationlimit of seismogenic thrusting at its margins. We estimate ~68 km shortening across theZagros Simply Folded Belt in the Fars region, and ~120 km total shortening of the Arabianplate. The Dezful Embayment is a low strain zone in the western Zagros. Deformation ismore intense to its northeast, in the Bakhtyari Culmination. The orogenic taper (acrossstrike topographic gradient) across the Dezful Embayment is 0.0004, and across theBakhtyari Culmination, 0.022. Lateral plateau growth is more pronounced farther east(Fars), where a more uniform structure has a taper of ~0.010 up to elevations of ~1750m.A >100 km wide region of the Zagros further northeast has a taper of 0.002 and iseffectively part of the Turkish-Iranian Plateau. Internal drainage enhances plateaudevelopment but is not a pre-requisite. Aspects of the seismicity, structure, andgeomorphology of the Zagros do not support critical taper models for fold-and-thrust belts.

Citation: Allen, M. B., C. Saville, E. J.-P. Blanc, M. Talebian, and E. Nissen (2013), Orogenic plateau growth: Expansionof the Turkish-Iranian Plateau across the Zagros fold-and-thrust belt, Tectonics, 32, 171–190, doi:10.1002/tect.20025.

1. Introduction

[2] The purpose of this paper is to examine the boundaryand relationships between the Turkish-Iranian Plateau andthe Zagros fold-and-thrust belt to its south (Figure 1),thereby improving our understanding of how and why suchorogenic plateaux grow in general. This subject is relevantto the wider issue of how and why the continents deformas they do. We show that plateau growth across the Zagrostakes place incrementally, that the present limit ofseismogenic thrusting does not coincide with a major bound-ary in the bedrock geology or a break in the regionaltopographic slope, and that plateau formation takes placeafter and to the northeast of the cutoff of thrustseismicity—implying that another process thickens the crust

at elevations higher than the seismogenic limit. These rela-tionships are relevant to the current debate as to whethercontinuum or microplate models are most appropriate forcontinental deformation, including the Arabia-Eurasiacollision [Reilinger et al., 2006; Liu and Bird, 2008].[3] Orogenic plateaux develop as a result of continent-

continent collision, as demonstrated by the Turkish-IranianPlateau within the Arabia-Eurasia collision zone [Şengör andKidd, 1979; Hatzfeld and Molnar, 2010]. A plateau can formduring continental collision where crustal thickening andsurface uplift are combined with relatively low erosion and in-cision rates that limit further thickening and relief generation[Liu-Zeng et al., 2008]. The Altiplano-Puna of South Americashows that orogenic plateau formation can also occur withoutcontinental collision, during oceanic subduction.[4] Elevated crust possesses more gravitational potential

energy than surrounding lowlands, leading to a buoyancyforce that resists further thickening and elevation of the pla-teau [England and Houseman, 1988]. Consistent with thisidea, the interior of the Turkish-Iranian Plateau is not under-going significant upper crustal shortening or thickening atpresent, indicated by the scarcity of active thrusts [Talebianand Jackson, 2004; Dhont et al., 2006; Allen et al., 2011a]and low internal strain, indicated by the Global PositioningSystem (GPS)-derived velocity field [Vernant et al. 2004].The simple explanation for the relationship between

1Department of Earth Sciences, University of Durham, Durham, UK.2Statoil, Oslo, Norway.3Research Institute for Earth Sciences, Geological Survey of Iran,

Tehran, Iran.4Department of Geophysics, Colorado School of Mines, Golden,

Colorado, USA.

Corresponding author: Mark B. Allen, Department of Earth Sciences,University of Durham, Durham, United Kingdom. ([email protected])

©2013. American Geophysical Union. All Rights Reserved.0278-7407/13/10.1002/tect.20025

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TECTONICS, VOL. 32, 171–190, doi:10.1002/tect.20025, 2013

seismicity and topography is that it is harder to continue toshorten crust that has already been significantly thickenedand elevated than it is to shorten the thinner crust towardthe foreland [e.g., Dalmayrac and Molnar, 1981]. Shearstresses are likely to be highest near gradients in crustalthickness [England and McKenzie, 1982], which has beeninvoked as the reason for the distribution of Zagros earth-quakes [Jackson and McKenzie, 1984].[5] What is not clear is whether such plateaux grow incremen-

tally or in discrete re-organizations [Tapponnier et al., 2001].Evidence is emerging that early deformation in collision zonescan occur thousands of kilometers from the original suture[Vincent et al., 2007; Clark et al., 2010], but this is not the sameas determining when compressional deformation and surfaceuplift cease in each area, i.e., the transition from an active fold-and-thrust belt into an orogenic plateau. Nor is it understoodwhether they grow by achieving a critical thickness and eleva-tion first in a limited area, which then laterally enlarges [Rowleyand Currie, 2006], or by awidespread rise in surface elevation topresent values. Such a rise could be protracted [Barnes andEhlers, 2009], or sudden, possibly as the result of loss of thelower lithosphere [Garzione et al., 2006].[6] Lithospheric thickness estimates for Asia based on shear

wave velocity gradients [Priestley and McKenzie, 2006] reveala core near the original Arabia-Eurasia suture with thicknesses

over 200km, thinning to near normal (100–120 km) values far-ther north and south. See also Kaviani et al. [2007].[7] Our approach to understanding the growth of the

Turkish-Iranian Plateau is to combine existing seismicityand GPS data (which provide the kinematic framework forthe study area) with new observations on the geology andgeomorphology. Sections 2.1 to 2.3 summarize the geologi-cal background of the Arabia-Eurasia collision and theZagros range in particular. We then provide a summary ofthe seismotectonics and GPS-derived velocity field, as theseprovide essential constraints on the location of active defor-mation. We utilize the seismicity data in constructing twonew crustal-scale cross-sections through the Zagros, basedon published geological maps, available sub-surface data,satellite imagery, and our fieldwork observations (section3). These cross-sections emphasize the along-strike variationin the structure of the Zagros, including (1) the nature of theDezful Embayment (Figure 2) and (2) the lower degree ofshortening in the High Zagros in the southeast. We examinethe geomorphology of the Zagros and the adjacent part ofthe Turkish-Iranian Plateau in section 4, as the tectonics ofa region may be recorded in its landscape. Digital elevationmodels, satellite imagery, and our fieldwork observationsshow the relationships of the landscape to the deep structureand the seismicity.

Figure 1. (a) Regional topography and seismicity of the Arabia-Eurasia collision. Large dots are epicen-ters of earthquakes of M >6 from 1900 to 2000 [Jackson, 2001], small dots are epicenters from the EHBcatalogue 1964–1999, M >�5. Red arrows show GPS-derived velocity with respect to Asia from Sellaet al. [2002]. A =Alborz; TIP = Turkish-Iranian plateau; Z =Zagros. (b) Seismicity of the Zagros: focalmechanisms reported in Nissen et al. [2011] and references therein. Note the scarcity of thrusts abovethe smoothed 1250m regional elevation contour (derived using a Gaussian filter with a radius of50 km). Earthquake epicenters are accurate to within 20 km [Nissen et al., 2011]. GPS vectors are fromWalpersdorf et al. [2006]. MZRF=Main Zagros Reverse Fault (Zagros suture).

ALLEN ET AL.: OROGENIC PLATEAU GROWTH: THE ZAGROS

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2. Regional Tectonics

2.1. Collision Overview

[8] The Bitlis-Zagros suture between the Arabian andEurasian plates follows a convex-northward line within SETurkey and runs NW-SE through northern Iraq and southernIran (Figure 1). South and southwest of the suture, the orig-inal passive continental margin of the Arabian plate is nowdeformed by folds, thrusts, and strike-slip faults within theZagros mountains [e.g., Jackson and McKenzie, 1984; Alavi,1994; Berberian, 1995; Agard et al., 2011; Mouthereauet al., 2012]. The present day deformation front lies approx-imately parallel to the Iranian shoreline of the Persian Gulf;Cenozoic folds occur within the northeastern Gulf, detectedon sub-surface seismic data [Soleimany and Sabat, 2010].The Gulf and the neighboring plains of Mesopotamia repre-sent the flexural foredeep to the Zagros fold-and-thrust belt[Beydoun et al., 1992; Aqrawi et al., 2010]. The originalEurasian margin lies along the southwest side of theSanandaj-Sirjan Zone, which is an elongate block of crustthat accreted to other blocks of crust within Iran during theCretaceous [Şengör et al., 1988].[9] The timing of initial Arabia-Eurasia collision is still

debated, with recently published estimates spanning from

the Late Cretaceous/Paleocene [Mazhari et al., 2009], theEarly Miocene [Okay et al., 2010] to the Mid/Late Miocene[Guest et al., 2006]. Allen and Armstrong [2008] summa-rized geological evidence from both sides of the suture thatindicate a Late Eocene (~35 Ma) age for initial collision,including a sharp reduction in magmatism and the develop-ment of unconformities on both the Arabian and Eurasianplates. Ballato et al. [2011] and Mouthereau et al. [2012]made the case for initial collision at ~35Ma followed byintensification of deformation at ~20Ma, perhaps causedby the end of underthrusting of thin Arabian margin crustunder Eurasia, and so the onset of deformation of the interiorof the Arabian plate [Morley et al., 2009].[10] This intensification is becoming more clear from a

combination of exhumation and sedimentology/provenancestudies [e.g., Fakhiri et al., 2008; Khadivi et al., 2010;2012; Gavillot et al., 2010; Rezaeian et al., 2012], but it isnot clear whether there was a smooth, incremental migrationof deformation and, potentially, plateau development towardthe foreland, or if there was a more discontinuous progres-sion in a series of abrupt re-organizations.[11] There was possibly also a re-organization of the

collision zone at ~5Ma [e.g., Axen et al., 2001]; many ofthe active fault systems within the collision zone seem to

Figure 2. (a) Location map and major structures of the Zagros Simply Folded Belt, Iran. Derived fromNIOC [1975, 1977], Berberian [1995], Hessami et al. [2001], Blanc et al. [2003], Agard et al. [2005], andBabaie et al. [2006]. Key to fault abbreviations: B =Borazjan; Iz = Izeh; K =Kazerun; KB=Kareh Bas;Kh =Khanaqin; S = Sarvestan; SP = Sabz-Pushan; BL =Balarud Line; A =Kuh-e Asmari. b) Earthquakeepicentres across the Zagros, from Nissen et al. [2011] and references therein, divided by fault type.MZRF=Main Zagros Reverse Fault.


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have originated or intensified at this time [Allen et al. 2004].There was no abrupt change in Arabia-Eurasia convergencerates at ~5Ma; if anything, convergence has slowed in thelast couple of million years [DeMets et al., 1994; Sellaet al., 2002]. Therefore, the re-organization may relate to adifferent cause, such as break-off of the Neo-Tethyan oce-anic slab [Keskin, 2003] or the closure of the free face atthe eastern margin of the collision zone [Allen et al., 2011a].[12] The Arabia-Eurasia collision actively deforms much

of SW Asia between western Turkey and eastern Iran, acrossan area of ~3� 106 km2 [Allen et al., 2004; Vernant et al.,2004; Agard et al., 2011; Mouthereau, 2011; Mouthereauet al., 2012]. Deformation is distributed across a broadregion from the Persian Gulf to the Caucasus, Alborz andKopet Dagh ranges, and between the Aegean and easternIran. Seismicity is concentrated at fold-and-thrust belts atthe margins of this region, particularly the Zagros in thesouth and the Greater Caucasus-Alborz-Kopet Dagh rangesin the north [Figure 1; Jackson and McKenzie 1984;Talebian and Jackson, 2004].[13] The Turkish-Iranian Plateau covers ~1.5� 106 km2,

mostly within the territory of Iran and Turkey (Figure 1).Most of the plateau area belonged to the Eurasian platebefore collision with Arabia. Elevations are typically over1.5 km, with subdued relief compared with the ranges to itsnorth and south. The geomorphic expression of the plateaucorresponds roughly with a division in the active tectonics:the precise correspondence is described later in this paper.Much of the plateau interior is seismically inactive, oraffected by strike-slip faults that have a variety of kinematicroles including strain partitioning with fold-and-thrust belts,

collision zone boundaries, and shortening by vertical axisrotations [Talebian and Jackson, 2002; Walker et al., 2004;Meyer et al., 2006; Dhont et al., 2006; Allen et al., 2011a].

2.2. Zagros Stratigraphy

[14] Stratigraphy in the Zagros records the evolution fromthe passive margin of the Arabian plate to the foreland basinof the Arabia-Eurasia collision zone (Figure 3), although asnoted above, the precise time of the onset of continentalcollision is debated. Precambrian basement is not exposedin situ but is recorded as blocks brought to the surface in saltdiapirs [Kent, 1979], derived from the Hormuz Series. TheHormuz Series is of late Precambrian/Cambrian age andoverlies the basement. The Series is present across a widearea of the Zagros and Middle East [Edgell, 1991] andcontains thick evaporites, mainly halite. The original distri-bution of these evaporites is not known. Some geologists[e.g., Murris, 1980; Edgell, 1991] infer that the presentdistribution of diapirs is a guide to the original depositionalextent: these occur across large areas of the eastern Zagros(Fars), but not within the Dezful Embayment or fartherwest (Figure 1). Hormuz Series salt also crops out in theHigh Zagros to the north of the Dezful Embayment. Areaswithout salt exposures were regions of clastic deposition ornon-deposition in such schemes. Other studies extend thedistribution of evaporites throughout the Zagros, withlater factors controlling the present distribution of diapirs[e.g., Kent, 1979].[15] The Hormuz Series lies at the base of a Palaeozoic plat-

form succession, similar to strata deposited over Pan-Africanbasement across North Africa and Arabia [Holt et al., 2010].

Figure 3. Stratigraphy of the Iranian Zagros. Modified from Iran Oil Operating Companies [1969] toreflect the diachronous nature of the Bakhtyari Formation [Fakhari et al., 2008].


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This is inferred to be interrupted by Permian rifting, which ledto the spreading of the Neo-Tethyan Ocean from thecontemporary margin of Gondwana [Şengör et al., 1988],although the evidence on which this is based remains sparse[Szabo and Kheradpir, 1978]. Sepehr and Cosgrove [2004]illustrate an example of a Permian-Triassic half graben fromthe Dezful Embayment. A ~4–5 km thick Mesozoic and lowerTertiary succession contains alternating deposits of clasticsand carbonates [Setudhina, 1978, Sherkati and Letouzey,2004]. Mid Cretaceous (Bangestan Group) and Oligo-Miocene(Asmari) limestones form prominent topographic markers inthe Zagros landscape, by virtue of their high mechanicalstrength and consequent low erosion rates.[16] Gachsaran Formation evaporites (Miocene) form a

second mobile unit in the stratigraphy, below a Miocene-Quaternary clastic succession that coarsens upward [Homkeet al., 2004]. Classical stratigraphies for the Zagros indicatea pulse of coarse conglomerates above an intra-Plioceneunconformity [James and Wynd, 1965]. Recent work showsthat these conglomeratic units are highly diachronous acrossthe range [Fakhari et al., 2008; Khadivi et al., 2010] andmost likely represent the switch to locally derived sedimentsin each area as deformation and relief proceed across thefold-and-thrust belt [Pirouz et al., 2011].

2.3. Zagros Tectonics

[17] The simplest tectonic scheme for the Zagros has twodivisions (Figure 2), southwest of the suture that is knownas the Main Zagros Reverse Fault. The High Zagros (orImbricate Belt) lies between the original suture and a majorthrust, the High Zagros Fault, which is mapped as runningroughly parallel to the suture and 40–160 km southwest ofit [Alavi, 1994; Berberian, 1995; Bosold et al., 2005]. Theremainder of the Zagros is the Simply Folded Belt.[18] The High Zagros is commonly depicted as a single

tectonic unit along the length of the range, but it is not[see, for example, Authemayou et al., 2006]. Nor does theHigh Zagros Fault have a similar character along the lengthof the range. In the northwest, the High Zagros consists oflow angle nappes that contain the following groups of lithol-ogies: (i) deepwater Triassic-Cretaceous clastics known asthe Radiolarite Group, (ii) Triassic-Cretaceous limestonesof the Bisotun Limestone, (iii) the Kermanshah ophiolitewith Late Cretaceous generation and emplacement ages,(iv) low grade metamorphics (phyllites) derived fromTriassic-Cretaceous sediments, (v) Barremian-AptianOrbitolina Limestones, and (vi) Eocene volcanics [Braud,1970; Ghazi and Hassanipak, 1999; Agard et al., 2005]. Thelast three sets of lithologies derive from the Sanandaj-SirjanZone, i.e., on the Eurasian side of the suture, and are overthrusttoward the southwest in one main sheet subdivided into threeunits [Agard et al., 2005]. The Arabia-Eurasia suture sensustricto (i.e., Main Zagros Reverse Fault) lies underneath thelowermost unit derived from the Sanandaj-Sirjan Zone. UpperOligocene-Lower Miocene conglomerates and limestones lieunconformably over the nappes and are themselves overlainbyMiocene flysch [Agard et al., 2005]. The High Zagros Faultis mapped as a low angle thrust at the base of a nappecontaining the Radiolarite Group [National Iranian OilCompany (NIOC), 1975].[19] Northeast of the Dezful Embayment, the higher

ground of the Simply Folded Belt and the High Zagros are

known collectively as the Bakhtyari Culmination. Highlyimbricated slices of Palaeozoic and Mesozoic strata withinthe High Zagros [Nemati and Yassaghi, 2010] are inferredto originate on the Arabian plate. There are outcrops of theHormuz Series salt along fault planes. Here there are thehighest summits (>4000m) and deepest erosion levels ofthe Zagros; rocks as old as the Cambrian are locallyexposed, and the regional exposure level is typically at theCretaceous Bangestan Group. The High Zagros Fault in thisarea is interpreted to cut the basement [Bosold et al., 2005;Authemayou et al., 2006] and overthrust to the southwest.[20] The Main Zagros Reverse Fault is more linear than to

the northwest and juxtaposes the Palaeozoic-Cretaceousstratigraphy against the low grade metamorphics andCretaceous cover sequence of the Sanandaj-Sirjan Zone[Authemayou et al., 2006]. Along strike to the southeast,within the Fars region, folds within the High Zagros becomemore open, indicating that the structure (and percentageshortening?) of the High Zagros is not the same along strike[NIOC, 1975; 1977]. Exposures of Palaeozoic strata andexposed thrusts both become rarer to the southeast. Thecommonest unit exposed is the Bangestan Group, but out-liers preserve Tertiary strata. The High Zagros Fault ismapped at the southwestern regional limit of the BangestanGroup but is not typically an exposed fault [Berberian,1995]. At the northeastern side of the High Zagros in Fars,there is the Neyriz ophiolite and associated melanges(Figure 2), thrust to the southwest over the Arabian platestratigraphy [Babaie et al., 2006]. These units are the equiva-lents of the Kermanshah ophiolite along strike to thenorthwest. They are adjacent to theMain Zagros Reverse Faultin this area, which is mapped as a steep thrust fault. There areno known low angle nappes derived from the Sanandaj-SirjanZone.[21] The greater part of the Zagros lies within the Simply

Folded Belt, where spectacular “whaleback” anticlinesdeform the sedimentary cover of the Arabian plate andexpose the Mesozoic-Cenozoic mixed carbonate-clasticsuccession. Palaeozoic strata are very rarely exposed. TheHormuz Series salt crops out in diapirs in the east ofthe Zagros [Gansser 1992; Talbot and Alavi, 1996], eastof the Kazerun Fault in the Fars region (Figure 2).Within Fars,there is a triangular zone tapering northward where there areno salt diapirs (Figure 2).[22] The regional southern limit of Asmari Limestone ex-

posure forms a distinct topographic front within the Zagros,referred to as the Mountain Front [McQuillan, 1991]. It hasbeen interpreted as marking the position of a major, albeitsegmented thrust front that runs along most of the Zagros:the Mountain Front Fault [Berberian, 1995; Cascielloet al., 2009]. In the southeastern Zagros, it splays off anotherdiscontinuous series of faults for which Berberian [1995]used the term “Zagros Foredeep Fault,” to refer to a blind“master” thrust that causes uplift along the Zagros foreland.[23] The Zagros has two embayment features, the Dezful

Embayment in Iran, and the similar but less well knownKirkuk (Kirkut, Karkuk) Embayment in Iraq [Figure 2;Carruba et al., 2006; Aqrawi et al., 2010]. These are definedby subdued relief and exhumation compared with zones alongstrike to the NW and SE. Thus, the Asmari Limestone is notexposed within the Dezful Embayment, except at Kuh-eAsmari, but crops out all around it. The origin of these


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embayments is not certain. Superficially, they resemble re-entrants in the front of a propagating thrust belt [Bahroudiand Koyi, 2003], but the Dezful Embayment does not repre-sent a significant change in the position of the frontal struc-tures of the Zagros: Compressional structures are presentwithin it, and the position of the deformation front is roughlylinear along the southwestern margin of the range (Figure 2).The high ground between the Dezful and Kirkuk embaymentsis known as the Pusht-e Kuh arc or the Lurestan arc.[24] Accommodation of roughly north-south plate conver-

gence is achieved by a combination of thrust and strike-slipfaults across the Zagros, in an example of strain partitioning.Right-lateral strike-slip along the Main Recent Fault trans-fers to the southeast into a diverging array of faults[Talebian and Jackson, 2002; Authemayou et al., 2009].[25] There are now many estimates for shortening across

the Zagros, based on balanced cross-sections. Some esti-mates put northeast-southwest shortening across the SimplyFolded Belt in the order of 40–60 km, and roughly consistentwhichever transect across the range is used [e.g., Blancet al., 2003; McQuarrie, 2004; Alavi, 2007]. There are somelower estimates that interpret very little faulting in theZagros cover sequence above the Hormuz Series: Sherkatiand Letouzey [2004] and Mouthereau et al. [2007]calculated 21 and 15 km of shortening across Fars, whileVergés et al. [2011] calculated 21 km for a transectacross the Pusht-e Kuh arc. Most estimates that includethe High Zagros only add ~20 km to these totals[e.g., McQuarrie, 2004].[26] Fewer estimates have been made for the amount of

underthrusting of Arabia beneath the Eurasian plate,deeper-level shortening of this underthrust crust, or shorten-ing within Eurasian units thrust over the Arabianplate. Vergés et al. [2011] used area balancing techniquesto suggest a total of 149–180 km shortening across theArabian plate. Mouthereau [2011] deduced 135 km of totalArabian plate shortening, by summing earlier estimates fordifferent parts of the Zagros. A portion of the thin, leadingedge of the Arabian plate has apparently underthrust Eurasiaand is detected on deep seismic profiles across Iran [Paulet al., 2010], as far as 170–270 km north of the surface traceof the suture. This range of values is distinctly higher thanthe 50 km of underthrusting calculated by Mouthereauet al. [2007] from structural restorations. Agard et al.[2005] found ~70 km of shortening adjacent to the suturein the northwest Zagros, where units originating in theSanandaj-Sirjan Zone overthrust the Arabia margin.[27] Teleseismic receiver function analysis provides infor-

mation on crustal thickness under the Zagros, although theprecise variation is debated. Paul et al. [2010] showed thatcrustal thickness is roughly constant across much of theZagros at 42� 2 km (moving southwest to northeast acrossthe strike of the range, starting from the undeformedforeland of the Persian Gulf region), to within a few 10s ofkilometers southwest of the Zagros suture. Farther northeast,the crustal thickness increases to 55–70 km, with the maxi-mum close to the line of the suture or the southwest part ofthe Sanandaj-Sirjan Zone immediately to its northeast. Rham[2009] described a more steady increase in Moho depth,from ~40 km near the southwest limit of the Zagros to~55 km at the suture. The Arabian plate southwest of theZagros has a crustal thickness of ~40 km [Gok et al., 2008].

2.4. Seismicity and Geodesy

[28] The northern promontory of the Arabian plate presentlyconverges with Eurasia at a rate of ~15–18mmyr�1, in anapproximately NNW direction [McClusky et al. 2000]. Con-vergence rates increase eastward, because the Arabia-Iran poleof rotation lies within the eastern Mediterranean region[Jackson and McKenzie, 1988] and is roughly 26mmyr�1 atthe longitude of eastern Iran [Sella et al. 2002] (Figure 1).These estimates are ~10mmyr�1 lower than the NUVEL-1Aplate motion model of DeMets et al. [1994].[29] Global Positioning System campaigns provide infor-

mation about the distribution of active strain within theZagros [Vernant et al., 2004; Hessami et al., 2006;Walpersdorf et al., 2006]. Active north-south convergenceacross the Zagros takes place at up to ~10mmyr�1, but thisis concentrated in the lower elevation parts of the range[Oveisi et al., 2009] and decreases northward toward theTurkish-Iranian Plateau. Internal deformation within the pla-teau, north of the Zagros, takes place at less than 2mmyr�1,which is roughly 10% of the overall Arabia-Eurasia conver-gence rate [Vernant et al., 2004].[30] Seismicity in the Zagros is concentrated between the

deformation front in the foreland and the regional 1250m con-tour [Figure 1; Jackson and McKenzie, 1984; Talebian andJackson, 2004;Nissen et al., 2011]. The fit is even better if onlythrust events are considered, because it leaves out strike-slipevents along the Main Recent Fault and along several of theright-lateral faults to the east of the Dezful Embayment.[31] Earthquake depths provide valuable information about

the style of local deformation and the strength profile of thelithosphere. Zagros earthquake studies confirm that thecrystalline basement deforms at up to depths of 20 km withearthquakes of M ~5–6, but no deeper [Baker et al., 1993;Maggi et al., 2000; Talebian and Jackson, 2004; Tataret al., 2004; Adams et al., 2009; Nissen et al., 2011]. Thereis some deeper microseismicity in a few areas, such as at Fin(Figure 2), where there is an abundance of aftershocks at20–25 km and a few as deep as 30 km, and near Kermanshah(Figure 2), where there are also a few well located events at~30 km [Nissen et al., 2011]. Note there is a rough equivalencebetween the uncertainty in the hypocenter depth (� 4km) andthe down-dip length of fault ruptured in events ofM ~5–6, i.e.,even if such an earthquake really occurred at 4 km above itsquoted depth, the fault would have ruptured to that depth,given the magnitude of the event. Some earthquakes must oc-cur within the sedimentary cover [Adams et al., 2009], giventhat their depths are >4 km above the likely depth to base-ment. Roustaei et al. [2010] and Nissen et al. [2007, 2010]suggested that the sequences of M ~5–6 earthquakes in Fin(in 2006) and Qeshm (in 2005–2009), both in the southeastSimply Folded Belt, were generated entirely within sedimentarycover. At least some of these events involved thrust faults thatdip southward, toward the foreland. Nissen et al. [2011] usedearthquake depth andmagnitude distributions across the Zagrosto propose that ruptures ofM ~5 events typically affected eitherthe sedimentary cover or the basement, but not both, consis-tent with the Hormuz Series salt forming an effective barrierto rupture propagation at the base of the sedimentary succes-sion. Rarer earthquakes of M ~6.5 produce ruptures so largethey must cut across the salt. These earthquake data haveguided our structural interpretation in Figure 4.


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[32] Low angle seismogenic thrusts are rare [Talebian andJackson, 2004; Nissen et al., 2011]. This does not rule outaseismic slip along detachments, which would be morelikely along the weak sedimentary units that act effectivelyas detachments in the first place.[33] The GPS-derived velocity field and the seismicity

record represent different aspects of the strain record acrossthe region. That they both show decadal-scale quiescence

across the higher elevation parts of the Zagros confirms theabsence of active upper crustal shortening within this region.

3. Structural Geology: Regional Cross-sections

[34] Figures 4a and 4b are cross-sections through theZagros, in the Dezful Embayment/Bakhtyari Culminationand Fars regions, respectively. They are based on

Figure 4. (a) Cross-section through the Dezful Embayment and the Bakhtyari Culmination. (b) Cross-section through Fars. Locations shown on Figure 2. (c and d) Restored and balanced sections for Figures 4aand 4b. Data are derived from SRTM topography [Jarvis et al. 2008], NIOC 1:1,000,000 maps [1975,1977], 1:250,000 maps of Llewellyn [1972, 1973], and satellite imagery from GoogleEarth. Although itis likely that most basement faults dip to the north, as shown, this is not conclusive in the seismicity data.


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combinations of 1:100,000 and 1:250,000 geological maps,satellite image interpretation, seismicity, fieldwork withinand at the margins of the Dezful Embayment, and reconnais-sance fieldwork in the Fars region. Debate continues as tothe sub-surface structure of the Zagros [Casciello et al.,2009; Farzipour-Saein et al., 2009; Vergés et al., 2011;Yamato et al., 2011]. The problem is that there are fewsub-surface data, especially for depths >10 km where mostcontroversy exists over fold and fault geometry. It is possi-ble to draw fold and fault geometries that are very differentat depth, based on the same surface and near-surface data.One end-member model emphasizes detachment of thecover from the basement, particularly along the HormuzSeries salt, but also on lateral equivalents where the evapo-rites are not known to exist [Stöcklin, 1968; McQuarrie,2004; Yamato et al., 2011]. The other end-member linkseach exposed fold to a basement thrust [Ameen, 1991]. It isentirely feasible that both thick-skinned and thin-skinnedstyles of deformation occur in the range, with multipledetachments within the sedimentary cover, co-eval with base-ment thrusts [Blanc et al., 2003; Ahmadhadi et al., 2007;Casciello et al., 2009; Mouthereau et al., 2006; 2007]. Thelatter are especially likely at zones of high structural relief,the so-called master faults of the Zagros [Berberian, 1995].[35] Seismicity clearly indicates that the basement actively

deforms with events of M ~5-6 [Baker et al., 1993; Talebianand Jackson, 2004; Tatar et al., 2004; Nissen et al., 2011]:Hypocenters of up to 20 km depth are located beneath thesedimentary cover, even allowing for uncertainty in the pre-cise depth of the hypocenter and the thickness of the stratig-raphy. We use this constraint to guide our interpretation ofFigure 4, locating a major basement thrust at features withparticular structural relief, such as the Mountain Front Fault[Berberian, 1995]. Other basement thrusts are likely, giventhe seismicity record, but it is hard to know their exact rela-tion to exposed folds; the locations of epicenters are notprecise enough.[36] There are also sufficient data to show that most

(~75%) earthquakes of M ~5–6 are located within the coverand do not cut across the Hormuz Series salt or any lateralequivalents [Nissen et al., 2011]. This constraint is also hon-ored in the cross-sections: Thrusts are blind and may not cutthrough carbonate anticlines exposed at the surface, but theydo exist under the Zagros Simply Folded Belt within thecover stratigraphy.

3.1. Dezful Embayment/Bakhtyari Culmination

[37] The section in Figure 4a covers the Dezful Embay-ment and higher terrain in the Bakhtyari Culmination to itsnortheast. The Dezful Embayment is a low relief, low eleva-tion region within the Simply Folded Belt, covering~75,000 km2. The origin of the Embayment may relate tothe pre-collision structural evolution of the Zagros, and thediscontinuous distribution of ophiolite nappes along itsnortheast margin: These are lacking adjacent to the Embay-ment, in contrast to the regions to the north and southeast[Allen and Talebian, 2011]. The Cenozoic sedimentary suc-cession reaches over 5 km in thickness [Koop and Stoneley,1982], and alluvial deposition continues across much of theEmbayment at present. However, it is not internallyundeformed but contains anticlines that act as hydrocarbontraps to Iran’s most important hydrocarbon fields

[Beydoun et al., 1992; Carruba et al., 2006]. These includethe Ahwaz anticline at the southwestern side of the Embayment(Figure 2). Seismicity occurs within the Embayment [Talebianand Jackson, 2004], and the fold geomorphology indicatesactive growth [Allen and Talebian, 2011], reinforcing thepoint about active deformation. The folds within the DezfulEmbayment typically do not expose the Asmari Limestoneor older units (Figure 4a), permitting the hydrocarbon systemsto operate without breaching of the traps.[38] Like most previous cross-sections for the Zagros

[e.g., Blanc et al., 2003; McQuarrie, 2004, Sherkati et al.,2006; Farzipour-Saein et al., 2009], we show little or nobasement relief where there is no change in structural eleva-tion on either side of anticlines. This is the simplest recon-struction but is an over-simplification given that the regionalstructure is affected by rifting during the Permian-Triassic[Sepehr and Cosgrove, 2004] and probably older and youn-ger intervals [Frizon de Lamotte et al. 2011]. In such cases,inversion of rift faults would involve the basement beneathanticlines but not produce significant structural relief athigher, exposed, levels of the stratigraphy.[39] Our interpretation depicts the folds as being strongly

linked to underlying thrusts. This view is strongly influencedby the seismicity record: It is hard to see how a regionaffected by numerous M ~5–6 events cannot be affected byfault slip (recalling that an event of M ~6 produces slip inthe order of 1m). Several papers model the Zagros folds asbuckle structures (e.g., Yamato et al., 2011), but thisapproach does not account for the seismicity.[40] The Gachsaran evaporites form a detachment

between overlying and underlying stratigraphy, and meanthat the uppermost parts of folds can be disharmonic withthe underlying structure [O’Brien, 1957; Sepehr et al.,2006]. Brittle thrusts deform this unit where it crops out onthe northeast side of the Dezful Embayment (Figure 5a)and place it over younger strata of the Agha Jari Formation.Such faults are depicted schematically on Figure 4a, whichdoes not attempt to represent the detailed structure withinand above the Gachsaran Formation. Other plausibledécollement units include the Cretaceous KazhdumiFormation (mudrocks) and the Triassic Dashtak Formation(evaporites) [Sepehr et al., 2006; Vergés et al., 2011].[41] An important structural step in this section line comes

at the Mountain Front Fault (Figure 5b), where the exposurelevel deepens to involve the Asmari Limestone, suggesting aregional elevation of basement on the kilometer scale[Berberian, 1995]. Anticlines to the northeast, within theremaining ~30 km of the Simply Folded Belt, expose theBangestan Group. Major anticlines within the SimplyFolded Belt are not adjacent to exposed thrusts but areinterpreted to overlie blind thrusts (Figure 4a).[42] The High Zagros Fault is the southwestern exposed

thrust associated with a major hanging wall anticline [Bosoldet al., 2005] and marks the boundary of the High Zagros.The High Zagros is at its narrowest where it is northeast ofthe Dezful Embayment (Bakhtyari Culmination), with awidth of ≤50 km. This contrasts with the Fars region, whereit is mapped as 100 km wide. There is much more imbrica-tion in the High Zagros, compared with the Simply FoldedBelt within and adjacent to the Dezful Embayment [NIOC,1975; Authemayou et al., 2006; Nemati and Yassaghi,2010]. Structural vergence is generally toward the southwest


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in the High Zagros; thrusts dip northeast. The presence ofHormuz Series salt along thrust planes and the lack of base-ment exposures suggest that the salt strongly decouples thecover and basement in this area. The spacing of thrustsdecreases toward the Main Recent Fault, which is an active,right-lateral strike-slip fault [Talebian and Jackson, 2002;Authemayou et al., 2009], roughly along the line of the orig-inal suture between the Arabian and Eurasian plates (MainZagros Reverse Fault). We depict the Main Recent Fault asdipping steeply, and cross-cutting inactive thrusts of theHigh Zagros at depth, based largely on the seismicity patternin the area [Nissen et al., 2011].[43] We find no evidence for two separate structural styles

or events in the folds, reported in the far east of the Zagros asevidence for a change from thin-skinned to thick-skinnedthrusting over time [Molinaro et al., 2005]. Therefore, sucha two-stage structural evolution is not considered further inour interpretation of the main part of the Zagros.[44] The northeastern limit of significant seismogenic

thrusting in this region lies within the Simply Folded Belt,to the southwest of the High Zagros Fault. There is no singlestructure associated with the cutoff, although earthquakesare concentrated in the region where topography climbsfrom the plains of the Dezful Embayment but elevationsare below 750m.[45] Figure 4c is a restored section for the line of Figure 4a.

The Simply Folded Belt is estimated to have shortened by~55 km or roughly 20%. This is line with previous estimatesfor the same region [e.g., Blanc et al., 2003; McQuarrie,2004]. Shortening for the entire Arabian plate is harder toconstrain, because of the highly deformed nature of the HighZagros and the presumed deformation of rocks underthrust

below the suture zone. We tentatively estimate shorteningfor the Arabian plate as far as the suture as ~110 km.Underthrusting of the Arabian plate beneath Eurasia andshortening within Eurasia need to be added to this value tocalculate total convergence in the collision zone.

3.2. Fars

[46] The structure across the Fars section line (Figure 4b)is much more uniform than the Dezful/Bakhtyari Culmina-tion transect, without an equivalent division between aregion of low relief/elevation/exhumation and a correspond-ing imbricate zone to the northeast [Mouthereau et al.,2007]. Although some equivalent structural units andboundaries have been defined and mapped [Berberian,1995], they have little in common with the Dezful/BakhtyariCulmination region depicted in Figure 4a. The MountainFront Fault is usually mapped at the southernmost exposureof Asmari Limestone, which is within 20 km of the modernshoreline of the Persian Gulf (Kuh-e Namak on Figure 4b).The most dissected anticlines along this line expose Creta-ceous limestones of the Bangestan Group. Some authorsplace the Mountain Front Fault ~100 km further north, alonga line of anticlines that also expose the Bangestan Group[e.g., Sepehr and Cosgrove, 2004]. This is at Kuh-eSurmeh along the section line of Figure 4b. There is noregional equivalent to the subdued folds and relief of theDezful Embayment. Berberian [1995] used this geomor-phic marker to suggest that the Mountain Front Faultwas originally continuous along the Zagros and is offsetby 10s of kilometers along the Kazerun Fault, at the east-ern side of the Dezful Embayment. However, detailedstructural studies along the Kazerun Fault reveal much

Bangestan Group

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Figure 5. Field photographs of Zagros structures. (a) Gachsaran Formation thrust southwest over AghaJari Formation clastics at the northeast side of the Dezful Embayment. Photo taken from ~32.03�N49.13�E. (b) View north of the Mountain Front Fault at Kuh-e Kamar Meh, taken from ~32.47�N49.15�E. Solid arrows mark terraces within Quaternary river gravels; open arrows mark the trace of theMountain Front Fault. (c) View west from a helicopter of Tang-e Chowgan (Shahpur), taken from29.8�N 51.65�E. Photograph courtesy of Peter Llewellyn. (d) View east of Kuh-e Zarghan, along the HighZagros Fault, taken from ~29.78�N 52.71�E.


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smaller offsets, on the order of 10 km [Authemayou et al.,2006; Lacombe et al., 2006]. Therefore, the southernmostappearance of the Asmari Limestone is a prominent geo-morphic marker along the Zagros, but not one that marksan original, contiguous fault zone. The term “MountainFront Fault” needs to be used with this caveat in mind.[47] Other folds within the Simply Folded Belt to the north

also expose the Asmari Limestone, and locally theBangestan Group. These are classic Zagros whaleback anti-clines, with half wavelengths in the order of 10 km and slightvergence toward the south (Figure 5c). As for the Dezfulregion, the deep structure of these folds is not clear, norhow they relate to underlying detachment(s) in the sedimen-tary cover and thrusts in the basement. Tatar et al. [2004]concluded that there need not be a one-to-one correlationbetween basement faults and observed folds, based on thespacing between seismic lineaments of 15–20 km and a cal-culated fold spacing of 10–15 km. (However, Mouthereauet al. [2007] reported an average wavelength of~16� 5 km for 149 Zagros folds, essentially the same as thisseismic lineament spacing.) Some thrusts may dip to thesouth, where there is little or no asymmetry in the overlyingfolds [Roustaei et al., 2010]. There is no obvious changein fold style northeast of the limit of seismogenicthrusting (Figure 4b).[48] The High Zagros Fault is conventionally mapped

within Fars as a thrust at the southern limit of the regionalexposure of the Bangestan Group [Berberian, 1995]. Thedistinction is not completely clear-cut. Lower Tertiary stratacrop out, albeit locally, northeast of the defined High ZagrosFault trace [e.g., NIOC, 1977], and as noted above, there areBangestan Group outliers in the cores of many of the anti-clines within the Simply Folded Belt. Nor is a thrust exposedin the region of the cross-section: The High Zagros Fault ispresumed to be blind. Our cross-section (Figure 4b) demon-strates that a basement thrust is a feasible solution for thestructure of the High Zagros Fault in this region, but not aunique solution. Its throw appears to be comparable withblind thrusts inferred beneath the Simply Folded Belt tothe south, i.e., in the kilometer range. The difference is thatan exhumation threshold has been crossed at the higher ele-vations in the north. Once the Asmari Limestone is removedby erosion, it is rare for a mantle of Lower Tertiary andUpper Cretaceous clastics to survive on the anticline crests;erosion removes these softer rocks until the resistantBangestan Group limestones are reached (Figure 5d).[49] Whereas the High Zagros in the Bakhtyari Culmina-

tion (Figure 4a) contains abundant thrusts and exposesPalaeozoic strata and Hormuz Series salt, the equivalentzone in Fars consists of whaleback anticlines that are identi-cal to counterparts in the Simply Folded Belt to the south(Figure 5d). Only the level of exhumation is different:deeper, but only on the scale of a kilometer or less. Largescale overthrusts and nappes of ophiolitic and/or suture zonerocks are not present until ≤30 km of the suture.[50] The northeastern limit of major seismogenic thrusting

lies within the Simply Folded Belt, up to 100 km south of thedefined line of the High Zagros Fault. It does not correspondto any individual structure within the Simply Folded Belt,but there is a good correspondence with the 1250m regionaltopographic contour [Jackson and McKenzie, 1984;Talebian and Jackson, 2004; Nissen et al., 2011].

[51] Figure 4d is a restored section through the Fars line ofFigure 4b. Northeast of Kuh-e Surmeh, the fold orientationis variable (Figure 2), which adds a degree of imprecisionto the restored section. Shortening across the Simply FoldedBelt is ~68 km or ~20%. The shortening value is higher thanthe Dezful Embayment/Bakhtyari Culmination line (55 km),although the percentage shortening is roughly the same. TheSimply Folded Belt shortening estimate is in line with manyprevious estimates for the Zagros [see summaries in Agardet al., 2011, Vergés et al., 2011, and Mouthereau et al.,2012], although much greater than those that assume littleor no faulting in the cover [Mouthereau et al., 2007; Vergéset al., 2011]. As noted, the seismicity record requiresfaulting in the sedimentary cover with earthquakes of M~5or above, and so we used this in our interpretation. Totalshortening of the original Arabian plate is hard to determinebecause of greater uncertainty in the structure at depth, closeto the suture zone.[52] We tentatively estimate ~100 km shortening of base-

ment across the entire Arabian plate, and ~120 km shorten-ing of cover; the difference is due to the Radiolarite Groupbeing stripped off its basement and transported farther south-west over the Arabian margin. These values are likewise inline with the restored section to the northwest but arealso speculative because little is well constrained about thesub-surface structure near the suture zone. This estimate doesnot include any wholesale underthrusting of the thin, leadingedge of the Arabian plate beneath Eurasia [Paul et al., 2010].[53] We have only interpreted basement faults in each

cross-section where there is a distinct change in structuralrelief on either side of an exposed structure (e.g., Kuh-eNamak and Kuh-e Surmeh), consistent with seismicity datathat suggest larger earthquakes in the Zagros occur inregions of greater structural relief, on faults that cut throughthe basement-cover boundary [Nissen et al., 2011]. This canbe seen as a minimum interpretation of basement involve-ment, but until more seismic data are available, it may notbe possible to make more detailed interpretations. In anycase, there are enough structural and seismicity data forbasement involvement to call into question models that treatthe Zagros as a critically tapered wedge above a basaldecollement at the level of the Hormuz Series [e.g., Ford,2004; McQuarrie, 2004]; it seems clear that as the basementis involved in the deformation, at very least, any criticaltaper model must include the basement [Mouthereau et al.,2006]. There may be a compromise, whereby typicalbasement thrusts have moved only so far as to juxtaposethe Hormuz Series rocks originally on either side of thePermian-Triassic rift. In the jargon of inversion tectonics,this is equivalent to saying that the null point of the invertedfault is at the level of the Hormuz Series.

4. Geomorphology

4.1. Topographic Profiles and Gradients

[54] Figures 6a and 6b are composite topographic profilesacross the Dezful/Lursetan and Fars regions of the Zagros,following the line of the cross-sections in Figure 4, andshowing the maximum, mean, and minimum elevations fora swath covering 50 km on either side of the central line.Elevations were obtained from version 4 of the ShuttleRadar Topography Mission (SRTM) dataset [Jarvis et al.


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2008]. SRTM data have a pixel size of 90m, and specifiedvertical absolute accuracy of ≤16m [Rodriguez et al.2005]. Specific vertical relative accuracy is quoted as≤10m [Rodriguez et al. 2005].[55] The interior of the Dezful Embayment is only a few

10s of meters above sea level. Average topographic gradientacross the Dezful Embayment is 0.0004, i.e., a very low oro-genic taper. Mean elevations were used for the calculation.Most of the elevation change (~2 km) in the Dezful/Bakhtyari Culmination section (Figure 6a) takes place north-east of the Mountain Front Fault, across ~100 km of thesection, as far northeast as the line of the Main Recent Fault.The Main Recent Fault is commonly a prominent depressionin topographic profiles across the Zagros, caused by the lo-calized erosion along the steep fault trace, and the presenceof pull-apart structures such as Borujerd [Talebian andJackson, 2002]. The orogenic taper along the profile inFigure 6a between the Mountain Front Fault and the MainRecent Fault is 0.022. The final part of the section containsthe highest elevations (typically ~2.5 km), and the drainagedivide. Between the Main Recent Fault and the drainage di-vide, the taper is 0.012, and from the divide to the end of thesection, it is 0.007, down to the northeast. Whereas theMountain Front Fault is a clear change in the topography,corresponding to the start of the high gradient section notedabove, the High Zagros Fault does not coincide with asignificant topographic step. Nor is there a change in topog-raphy at the limit of seismogenic thrusting, either at the

regional ~1250m threshold or the local limit of <500malong the topographic profile.[56] The Fars profile (Figure 6b) shows a steady climb

from the coast up to mean elevations of ~1750m at~180 km inland (250 km along the profile), with a taper of0.010. The rest of the profile as far as the drainage divideat 425 km has a lower taper of 0.002; i.e., the mean eleva-tions show a plateau topography. The difference betweenthe maximum and minimum elevations decreases to thenortheast of ~250 km along the profile, reflecting alluvialinfilling of the synclinal valleys. Neither the High ZagrosFault nor the Main Zagros Reverse Fault corresponds to apronounced topographic step; the change over from thehigher to lower taper occurs southwest of the High ZagrosFault, and northeast of the 1250m elevation contour thatroughly marks the limit of seismogenic thrusting. The transi-tion between the two parts of the section lies within the Sim-ply Folded Belt and does not correspond to any major struc-ture mapped at the surface. The swath includes externallydrained regions as far as the High Zagros Fault, hence thesteady climb in minimum elevation to this point.[57] The maximum value profile in Figure 6b picks out the

peaks of individual fold crests within both the SimplyFolded Belt and the High Zagros. These are largelysmoothed out in the mean value profile, because the swathwidth is greater than the length of any individual fold. Acouple of folds are so long that their crests perturb the meanand minimum profiles.[58] The rise in average elevations north of the

seismogenic thrust limit (Figure 6b) occurs up to roughlythe regional 1750m elevation contour. It presumablyinvolves an isostatic response to crustal thickening, but theprecise mechanism for this thickening is literally cryptic:There is no signal from seismicity or available GPS cover-age [Hessami et al., 2006; Walpersdorf et al., 2006; Nissenet al., 2011] (Figure 1b). We return to this issue in section 5.[59] To understand the topographic signatures in more

detail, we have analyzed the variation in gradient and eleva-tion across the Zagros, across the structural strike through boththe Dezful Embayment and Fars regions. Data are derivedfrom the SRTM v4 dataset [Jarvis et al. 2008].[60] Figure 7 shows the gradients for the Dezful Embay-

ment and adjacent areas of the Zagros, derived from theSRTM digital elevation data and processed using ArcGISsoftware. Low gradients across the Embayment (generallyequivalent to slopes of <1�) contrast with higher gradients(>0.1, roughly equivalent to >10� slopes) within theBakhtyari Culmination to its northeast, both within areasdefined as the Simply Folded Belt and the High Zagros. Thisis a different pattern from that in either the Pusht-e Kuh arcto the northwest or the Fars region to the southeast. In bothof these areas, a more consistent pattern of slopes appliesfrom the deformation front to the northern limit of the HighZagros: High slopes over the spaced anticlines are separatedby low slope regions of the synclines. The synclines are typ-ically occupied by rivers, either draining externally ordammed. The valleys tend to be wider in the High Zagrosthan the Simply Folded Belt, both in the Pusht-e Kuh arcand in Fars (Figure 7). The strong carbonates exposed onthe anticline flanks maintain high slopes, such that there isno discernible reduction in slope between the seismicallyactive and inactive parts of the range (Figure 7).

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[61] Figure 8 shows normalized frequency histograms forgradients within windows in the Turkish-Iranian Plateauand southwest across the suture in to the Zagros range. Dataare shown in gradient divisions of 0.05. Four windows areused for two transects, across the Dezful Embayment/Bakhtyari Culmination and Fars; locations are shown onFigure 7. Window size is varied to make sure windows lieentirely within one or other of the zones of interest. There-fore, the windows northeast of the suture are 1� � 1�, whilethose to the southwest are either 0.5� � 0.5�, or 0.25� � 0.25within the narrow Bakhtyari Culmination.[62] Results for the Dezful/Bakhtyari Culmination

windows, D1–D4, are shown in Figure 8a. There are lowgradients within the Embayment itself (D4), as expected.Both the windows between the Mountain Front Fault andthe Main Recent Fault (D2 and D3) show a roughly sym-metrical distribution of values around a highest frequencygradient of 0.5–0.6. Not only are there high summits andsteep slopes in these windows, but there are not significantplains along the valleys. Gradient distributions in regionD3 (Simply Folded Belt above 1250m) are intermediatebetween the distributions of D2 (High Zagros) and D4(Dezful Embayment).[63] Plateau morphology is only established northeast of

the Main Recent Fault, where the frequency peak is at gradi-ents of 0.05 (D1). We interpret these results to mean thatcrustal thickening has occurred northeast of the MountainFront Fault, producing the steady increasing elevation. Themechanism is addressed in section 5.[64] Results for the higher elevation Fars windows north

of the limit of seismogenic thrusting (F1–F3) show peaks

in frequency at gradients of 0.05 for each window(Figure 8b), with the frequency at this peak increasing fromsouthwest to northeast. The lowest elevation window, withinthe seismogenic thrust sector of the Simply Folded Zone(F4), shows a frequency peak for gradients of 0.1, and apositively skewed distribution.[65] The relief in the Fars region becomes more subdued

beyond the limit of seismogenic thrusting. In this higherarea, many of the synclinal basins are being infilled byalluvium, leading toward the construction of plateaugeomorphology, including regions that are geologically partthe Simply Folded Zone.

4.2. Drainage Patterns

[66] In this section, we look at the form of the main drain-age basins that cover the Zagros, to see how they relate tothe deformation patterns. Mouthereau et al. [2007] andRamsey et al. [2008] have analyzed the interactions betweenrivers and individual folds in the Zagros. Burberry et al.[2008] presented a comprehensive map of drainage withinthe east of the Zagros.[67] There are three main drainage basins in the Iranian

Zagros (Figure 9) that drain through the rivers Karen, Mand,and Kul, and several smaller ones along the coast, such asZohreh and Helleh [Oberlander, 1965]. Three internallydrained areas occur within the Fars region: Niriz, Shiraz,and a collection of smaller basins we informally call Razak[Mouthereau et al., 2007; Walker et al., 2011], after thesettlement within the area.[68] Rivers in the Karun drainage basin (Figure 9)

converge on the Dezful Embayment and join the Karun

100 km

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Figure 7. Gradient map for the Zagros, derived from the SRTM v4 dataset, using averages of 1 km win-dows. Highest gradients are predominately in the Bakhtyari Culmination.


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River either within the Embayment or beyond its southwestboundary (Figure 9). The Karun River joins the ShattAl-Arab ~80 km southeast of the confluence of the Tigris

and the Euphrates. The latter two rivers are the axial drain-age to the Zagros fold-and-thrust belt. The drainage patternwithin the Dezful Embayment is consistent with its present

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a

b

Figure 8. Normalized cumulative frequency plots for gradients of 1� 1 km windows within the DezfulEmbayment/Bakhtyari Culmination (a; regions D1–D4) and the Fars region (b; regions F1–F4). Data arederived from the SRTM v4 dataset.


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low altitude, and its existence as a depocenter back to theOligocene [Koop and Stoneley, 1982; Motiei, 1993].[69] The northwest boundary to the Karun drainage basin

is in the region of the Khanaqin Fault (Figure 2). The bound-ary between the Karun, Niriz, and Mand drainage basins liesclose to the north-south Kareh Bas and Kazerun faults(Figure 2). Therefore, both of these prominent drainagedivides occur at important structural boundaries within theZagros. In both cases, the region of the drainage divide ismarked by a change in relief, with higher ground and deeperexposure level to the east (Figure 9; NIOC, 1977).[70] The Mand and Kul drainage basins are roughly equal

in size and proportions (Figure 9). They are separated by theRazak region of internal drainage, roughly 10,000 km2 inarea [Mouthereau et al., 2007; Walker et al., 2011]. Thisregion may have become isolated and through-going drain-age defeated as the result of fold growth in the south [Walkeret al., 2011]. Walker et al. [2011] and Khadivi et al. [2012]noted that sediment ponding would act as a process to raiseaverage elevations in the lower elevation regions of inter-nally drained basins and so encourage plateau development[Sobel et al., 2003]. Note that the Razak region lies belowthe regional 1250m contour and within the area affectedby seismogenic thrusts.[71] There is less evidence of antecedent drainage in the

Mand and Kul systems than in the Karun drainage basin,witnessed by the greater sinuosity of the Mand and Kuland their tributaries. There are more places where the riversare diverted around the rising tips of folds [Ramsey et al.,2008]. This is possibly because discharge rates and sedimentflux are lower in the Fars region than the Dezful Embay-ment/Bakhtyari Culmination [Oberlander, 1965], andtopographic gradients are lower (section 4.1), such that the riv-ers lack sufficient stream power to cut through rising folds.[72] The Iranian coastline of the Persian Gulf lacks major

deltas southeast of the Tigris/Euphrates system and is

relatively sediment starved, even at the mouths of the Mandand Kul Rivers (Figure 9). This in contrast to the DezfulEmbayment, where progradation of the Karun has rapidlychanged the shoreline of the Gulf [Heyvaert and Baeteman,2007], possibly at rates of 20m/a between 5000 and 2500 BP.[73] Small drainage basins occupy the coastal strip of the

eastern Zagros, enhancing the sediment starvation of thiscoast. Between them and the Mand and Kul drainage basins,there is an elongate drainage basin, occupied by the MehranRiver, which flows west to east between two prominentanticlinal ridges for ~300 km [Figure 9; Ramsey et al.,2008]. The relative timings of drainage evolution are notcertain, but it is likely that the Mehran drainage basin is alate stage feature, given its position near the frontal struc-tures of the Zagros.

4.3. Landscape Evolution

[74] In section 3, we highlighted how the limit ofseismogenic thrusting corresponds with the regional1250m elevation contour, but not a distinctive structuralboundary at exposed levels. In section 4.1, we showed thatthe topographic profiles across the Zagros do not closelyreflect this division between thrusting and non-thrustingparts of the range, with high regional topographic gradientsbeyond the limit of seismogenic thrusting. In this section,we describe local patterns of slope and incision in differentparts of the Zagros.[75] The development of plateau morphology in the Fars

region is reflected in local patterns of deposition and incision(Figure 10a). Anticlines within the actively thrusting part ofthe Simply Folded Belt are associated with alluvial fans ontheir flanks (Figure 10b). These fans are traversed by braidedriver channels, which may be incised. Some anticlines areflanked by bajadas (Figure 10c). There is a lithologicalcontrol on whether flanking fans are discrete or merged.Fold limbs that expose one or the other of the Asmari orBangestan carbonates tend to be flanked by bajadas, formedfrom numerous small, parallel streams that flow down thefold limb. Kuh-e Mehmand is an example (Figure 10c).Where the Asmari Limestone has been breached but stillforms the main topographic slope, drainage upstream ofthe Asmari Limestone outcrop coalesces into systems withenough stream power to penetrate the Asmari Limestoneridge that faces the fold axis (e.g., Kuh-e Namak,Figure 10b). Such rivers form wider-spaced drainage atlower elevations on the fold flanks.[76] Rivers in the internally drained basins terminate in

lakes such as Daryacheh-Ye Maharlu, 20 km southeast ofShiraz (Figure 10a). Significant lengths of the margins tothese basins lack transverse alluvial fans. Sedimenttransported by axial rivers is aggrading, filling the basintopography. The anticlines are effectively being buried bythe clastic sediments, even though the resistant carbonatelithologies maintain steep slopes (Figures 10d–10e).[77] Our interpretation of this variation in landscape is that

the alluvial fans on the frontal folds in the Zagros, which areactively deforming and undergoing surface uplift, are proneto tilting, incision, and cannibalisation by active river chan-nels. Once an area has ceased seismogenic thrusting, it issmoothed out by erosion, leading first to the bajadas and inthe final stage to the wide valleys and lakes within the HighZagros, without axial fans on the flanks of the subdued fold

Figure 9. Drainage patterns of the Zagros. Note the inter-nally drained areas both outside and within the zone ofseismogenic thrusting.


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topography. In this way, the lateral tectonic growth of theTurkish-Iranian Plateau toward the southwest is followed byits geomorphic growth. There is no fixed rate at which the timelag occurs, partly because there is a dependency on whether aregion in the High Zagros keeps an efficient external drainageconnection through to the rest of the range and the coast, or if itbecomes internally drained [Walker et al., 2011].

5. Discussion

[78] Several papers have addressed the topographicprofiles across the Zagros in terms of implications for under-lying detachment horizons, especially along the HormuzSeries at or near the basement/cover interface [Ford, 2004;McQuarrie, 2004; Mouthereau et al., 2006; 2007]. The

100 km

54 E52 E30 N

29 N

28 N

HZigh agros

Si ply FBe t

m olded l

Shiraz

Jahrom

Firuzabad

H gh Za ro Fa l

ig

su t

MZR

F

a

b

b

c

d

c

d eNS

Daryacheh-Ye Maharlu

Figure 10. (a) Landsat (MrSID) mosaic of the Fars region (bands 7, 4, 2). (b) Close-up of Kuh-e Namak.Arrows highlight stream channels that have breached the carapace of Asmari Limestone. (c) Example ofbajadas at the foot of ridges where the carbonate carapace is intact. (d) Drowned anticlines in the HighZagros: Valley floors are filling with alluvium in the internally drained Niriz basin. (e) View west ofPersepolis (Takht-e Jamshid), showing alluvial plains beneath a dissected anticline of Bangestan Grouplimestones. The anticline has an open interlimb angle, typical of the strain in this part of the High Zagros,despite surface elevations of >1500m.


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width (~300 km) and elevation drop (~2 km) across the Farssector of the Zagros combine (Figure 6b) to give it a low av-erage gradient, previously interpreted in terms of an efficientHormuz salt detachment allowing effective propagation ofthrusting toward the foreland [Ford, 2004]. The problemwith such interpretations lies in treating the Zagros as thoughthere is a single gradient that applies across the range. This isnot the case for either swath shown in Figure 6.[79] An alternative explanation is as follows. Seismogenic

thrusting takes place at elevations <1250m, and associatedcrustal thickening produces the topographic gradientbetween sea level and this elevation [Jackson and McKenzie,1984; Nissen et al., 2011]. Seismogenic thrusting ceasesonce the elevation gain is sufficient to generate enoughgravitational potential energy to resist additional shortening[Allen et al., 2004]. GPS-derived velocity field data givesupport to this argument, as velocities drop off well south-west of the High Zagros (Figure 1b), although the currentdensity of available data is not high enough to compare pre-cisely to the seismicity and topography signals [Hessamiet al., 2006; Walpersdorf et al., 2006]. But another processmust be going on, as average elevations climb to ~1750min Fars, and ~2500m northeast of the Dezful Embayment(Figure 6). If the extra elevation is a consequence of crustalshortening and thickening, via isostasy, part of this must happenby aseismic processes. However, low GPS-derived velocities,relative to central Iran, across the higher parts of the SimplyFolded Belt and the High Zagros [Hessami et al., 2006;Walpersdorf et al., 2006] make this unlikely to be happening ac-tively at upper crustal levels, in keeping with the reduction inthrust seismicity above the 1250m regional elevation contour.[80] We conclude that there is a contribution from

aseismic and/or ductile crustal thickening of the basementbeneath the higher parts of the Simply Folded Belt and theHigh Zagros, consistent with the increase in crustal thick-ness as the suture is approached from the southwest [Paulet al., 2006; 2010; Rham, 2009; Mouthereau et al., 2012].[81] Underneath the suture zone and the Sanandaj-Sirjan

Zone, high elevations may relate to underthrusting of theleading edge of the Arabian plate, with or without additionalinternal thickening of the crust [Mouthereau et al., 2007;Paul et al., 2010; Agard et al., 2011]. Southwest of thesuture such underthrusting is not geometrically feasiblebecause the region is within the Arabian plate. The samegeometric argument applies to the idea that break-off ofthe subducted oceanic Neo-Tethyan slab could have causedsurface uplift: This may apply north of the suture in regionssuch as eastern Turkey (given that any such break-off islikely to be a geologically recent phenomenon, van Hunenand Allen, 2011) but is unlikely to the south.[82] Ductile shortening of the Arabian plate basement is a

more plausible explanation of the elevations above theseismogenic limit of thrusting. This extra elevation above~1250m is effectively a topographic signal of depth-depen-dent strain during mountain building and plateau construc-tion, and approximates to pure shear under the higher partsof the Zagros. It is analogous to models of depth-dependentcontinental extension, where different values for the exten-sion factor, b, determined by upper crustal and whole crustalmethods have been used to infer greater strain in the weaklower crust compared to the strong upper crust [e.g., Driscolland Karner, 1998]. There is a further similarity to models of

continental extension [e.g., Jackson and White, 1989], inthat the Zagros need not be underlain by a single low-anglethrust, detaching everything above it from underlyingbasement. Instead, individual thrusts may tip out at the seis-mic-aseismic transition, passing into a region of diffuse,ductile deformation.[83] We rule out a flexural effect from loading in the

suture zone: The effective elastic thickness of the Arabianplate is ~15 km [McKenzie and Fairhead, 1997], which ismuch lower than the distance between the limit ofseismogenic thrusting and the establishment of a low taperin the Fars region (~40 km, Figure 6b).[84] Across much of the Turkish-Iranian Plateau farther

north the elevated crust is neither especially thick noractively shortening [~42 km across central Iran north of theSanandaj-Sirjan Zone; Paul et al., 2006]. Support from man-tle density variations may be a factor promoting the regionalelevation in this region: Maggi and Priestley [2005] usedsurface wave tomography to show an area of low velocitiesin the upper mantle to depths of ~200 km below the Turkish-Iranian Plateau. The widespread, if patchy, Quaternarybasaltic magmatism across the plateau implies partial melt-ing, consistent with this idea [e.g., Pearce et al., 1990;Kheirkhah et al., 2009]. Such mantle may support elevatedtopography, above what is generated by the crustal thicknessalone. However, Allen et al. [2011b] showed that some partsof the plateau near the Turkey/Iran border are characterizedby relief that essentially pre-dates the Quaternarymagmatism in the region, and slow rates of incision sincethe lavas flowed down the valleys (0.01–0.05mmyr�1).[85] The cutoff in seismogenic thrusts in the Zagros occurs

at lower elevations (~1250m) than in the Himalayas andTibet [~3.5 km; England and Houseman, 1988], and themaximum elevations of the Zagros and the Turkish-IranianPlateau are also much lower (~2 km regional elevation com-pared with ~5 km for Tibet). It is not completely clear whythis is the case. It may relate in some way to the strengthof the crust in each area, with both the Arabian plate and Iranbeing weaker than India and Tibet [England and Houseman,1988; Maggi et al., 2000]. There is a striking similaritybetween both plateaux, notwithstanding the differences inabsolute values of elevation: In both regions, there is aregional gain in elevation above the seismogenic limit ofthrusting, implying that another process is involved. InTibet, this extra elevation has been explained by loss of thelower lithosphere and consequent isostatic uplift [Englandand Houseman, 1988]. However, recent work on the deepstructure of Tibet indicates that a thick lithosphere is in placeunder much if not all of the plateau [Priestley and McKenzie,2006; Chen et al., 2010], with obvious implications for de-lamination models. Nor can underthrusting of the Indianplate be responsible for the excess elevation at all the plateaumargins: It is not simply a phenomenon confined to alinear front across Tibet. We therefore speculate that depth-dependent crustal shortening is responsible for the extraelevation of the Tibetan plateau, above the 3500m limit ofseismogenic thrusting.[86] New crustal-scale cross-sections help define the

regional structure of the Zagros. In the eastern Zagros (Fars),the whole fold-and-thrust belt is ~300 km across strike andhas a similar structural style throughout, albeit with a deepererosion level in the High Zagros than the Simply Folded


186

Belt. In the northwest Zagros (Lurestan/Khuzestan prov-inces), the fold-and-thrust belt is narrower and includes alow-deformation zone within the southwest of the SimplyFolded Belt: the Dezful Embayment. This region hasresisted late Cenozoic deformation better than its surround-ings and acts as a major depocenter. Deformation is corre-spondingly more intense northeast of the Dezful Embaymentin the Bakhtyari Culmination, presumably to maintainroughly constant strain across the Zagros as a whole. Thisis reflected in the geomorphology of this area, with thehighest elevations and steepest slopes in the whole of theZagros (Figures 7 and 8). As a consequence of the DezfulEmbayment, lateral plateau growth is more limited at theselongitudes than to the east in Fars, where plateau growthhas migrated furthest south in the Zagros.[87] Several factors in Fars create low exhumation rates,

which lead to more rapid crustal thickening for any givenshortening, and therefore crust that more rapidly reachesthe elevation threshold at which seismogenic thrusting stops.These factors are as follows: a semi-arid climate and conse-quent low erosion rates; a regional salt detachment and/orinherited basement faults—both of which encourage defor-mation to propagate toward the foreland; and resistant car-bonate lithologies at both Tertiary and Cretaceous levels,which impede erosion and exhumation.[88] These ideas are summarized in Figure 11, which

shows seismogenic thrusting occurring in a belt across theZagros that at any one time is confined to the region below~1250m elevation. As crustal thickness builds up, thelocation of this belt migrates toward the foreland, over timeincluding areas previously unaffected by significant thrusting.[89] There is no abrupt change in the landscape of the

Zagros to match the cutoff in active, seismogenic thrusting.But there is a reduction in topographic gradient across thehinterland parts of the range, and a tendency to form inter-nally drained basins. Sediment filling these basins enhancesthe smooth topography in these regions, as anticlines aregradually buried in alluvium [Walker et al., 2011]. Even inareas with external drainage, the semi-arid climate keepsthe rivers relatively small, while limestone gorges chokeoff sediment flux and pond it within synclinal basins. Suchinefficient erosion and sediment transport reduces theaverage gradient of the Zagros, even though summits ofcarbonate-cored anticlines resist erosion and maintain highrelief above the surrounding plains.[90] The way in which seismogenic thrusting relates to

topography in the Zagros, rather than major structural bound-aries, is more in keeping with models of tectonics that high-light its diffuse nature [continuum approach, e.g., Liu andBird, 2008], rather than microplate models [e.g., Reilingeret al., 2006]. Microplate models may honor the kinematic databut do not capture the incremental changes in the distributionof strain over time, nor the possibility that lower and uppercrustal deformation are decoupled. By its very nature,aseismic, probably ductile, basement strain is difficult todetect. Topographic signals, such as the elevation climb ofthe Zagros detailed in this paper, may be one of the bestinsights in areas of active deformation.[91] The structure of the Zagros is also very relevant to

critical taper models of fold-and-thrust belt growth [Dahlen,1990] but raises some problems: Steep thrusts that are pres-ent and active across wide areas of the range and involve

basement do not easily fit the idea of a single, low-angledetachment at the base of a deforming zone. Nor is there asingle topographic slope that applies across any given cross-section; rising topography beyond the limit of seismogenicthrusting implies a decoupling of upper crustal (brittle) andlower crustal (ductile) deformation, but one that is ultimatelylimited by the buoyancy force associated with elevated crust,rather than a requirement to maintain a critical angle for anentire orogenic wedge.

[92] Acknowledgments. We thank the Geological Survey of Iran fortheir support. Part of this work was started when MBA and EJB were atCASP, Cambridge. This work was supported by the Natural EnvironmentResearch Council [grant number NE/H021620/1]. We thank OlivierLacombe, an anonymous reviewer, and the editors for their helpful com-ments on the first version of the paper.

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MohoSeismogenic zone

Seismogenic zone

Incremental foreland propagation of seismogenic thrusting

Seismogenic thrusts end at 1250 m elevation contour

Zagros Simply Folded Belt

Zagros Suture/ Main Recent Fault

High ZagrosTimestep 1

Timestep 2 (Present)

0 40 km

V = H

Sedimentary cover

Major earthquake

Seismogenic thrust

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sea level

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SW NE

Ductile basement shortening continues after upper crustal deformation stops

Surface uplift continues after thrusting stops

Limit of surface uplift reached: this part of the Zagros forms part of the Iranian Plateau

Iranian Plateau

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Durham Research Online - COnnecting REpositories · 2013-09-13 · Zagros Simply Folded Belt in the Fars region, and ~120km total shortening of ... et al., 2012]. The present day